The present invention relates generally to current sensing devices for electrical systems, and more particularly to automatically resettable alternating current fault indicators.
With the increased use of underground electrical distribution systems wherein primary and secondary feeder cables are directly buried in the ground, the need has arisen for an improved detection apparatus for determining the location of a short circuit or fault in the system cables or associated electrical components. Since the cables are buried, it is often difficult to determine the exact location of a fault, particularly where there are several distribution transformers, each serving multiple customers.
Cables of the type used in underground distribution systems generally consist of a central conductor covered by an insulating layer and an electrically conductive rubber ground sheath. Detection of fault currents in such cables is most readily accomplished by means of clamp-on type fault indicators, such as those manufactured by E. O. Schweitzer Manufacturing Co. of Mundelien, Illinois. Such fault indicators may be either of the manually reset type, wherein it is necessary that the indicator be physically reset following each fault, or of the automatically reset type, wherein a fault indication is reset upon restoration of line current. Examples of such manually and automatically reset fault indicators are found in U.S. Pat. Nos. 4,086,529, 3,676,740 and 3,906,477, of the present inventor. of the two types of fault indicators, the automatically reset type is better adapted for use in underground distribution systems in that it is not necessary to gain access to the fault indicator following a fault.
One drawback of self-resetting clamp-on type fault indicators is that they require a source of excitation for powering their automatic reset circuitry. Heretofore, this has been provided by either an inductive coupling to the conductor in which current is being sensed, or by means of a connection to an appropriate external excitation source associated with the distribution system, such as the secondary winding of a distribution transformer. The advantage of the interconnection method over the induction method is that the power derived by the fault indicator is not dependent on current flow in the conductor. Unfortunately, the use of a metallic connection for this purpose would subject the distribution system to being short-circuited should the wire connection be damaged or otherwise grounded. Moreover, a lineman inspecting or servicing the fault detector would be subject to the danger of electric shock should he inadvertantly come into contact with the metallic connection or the interior circuitry of the fault detector.
Accordingly, it is a general object of the present invention to provide a new and improved interconnection means for a resettable fault indicator whereby the fault indicator can be energized from an external power source without the danger of electric shock or introducing an electrical short circuit to the system.